SoulCalibur II HD Online will render the classic game in high definition graphics and will allow online play. The latest gameplay trailer confirmed the return of SCII's PS2-exclusive guest character, Tekken's Heihachi Mishima.
The most recent update confirms that Todd McFarlane's Spawn, the guest character for SCII's Xbox version, will also make a comeback.
The "guest characters" tradition
SoulCalibur IV included Darth Vader, Yoda and The Apprentice/Galen Marek/Starkiller from the Star Wars series; the PSP's SoulCalibur: Broken Destiny had God of War's Kratos; and SoulCalibur V featured Assassin's Creed's Ezio Auditore da Firenze and the moveset of Tekken's Devil Jin, which you can unlock for the character creation mode after defeating the in-game AI of Tekken God Katsuhiro Harada.
Sources: The Fighters Generation, VG 24/7